1. Field of the Invention
The present invention relates to a magnetic memory device and a method of reading information, and in particular, to, for example, the circuit configuration of memory cell through read bit lines through in a magnetic memory device and a method of reading information from a magnetic memory device.
2. Description of the Related Art
Each cell in a magnetic random access memory (MRAM) using a tunneling magneto resistive (TMR) effect is composed of an magnetic tunnel junction (MTJ) element and a selection transistor having one end connected to the MTJ element and the other end connected to a ground potential line.
Information can be read from a magnetic memory device at a low voltage utilizing a good MR ratio. However, a sense amplifier is composed of a conventional n-type Metal Oxide Semiconductor (MOS) transistor or CMOS transistor, so that as the power voltage decreases and the bit line voltage correspondingly approaches its threshold, information may not be read at high speed or read operations are disabled. Thus, a mechanism is required which increases the amplitude of the sense amplifier with respect to the magnitude of a read signal, that is, the amplitude of bit lines. Such an overdrive technique for the sense amplifier is described in “A 0.9V Sense-Amplifier Driver for High-Speed Gb-Scale DRAMs”, K. Gotoh et al., Symposium On VLSI cir., June, 1996, and “Low Voltage High Speed Circuit Designs for Giga-bit DRAMs”, K-C Lee et al., Symposium On VLSI Cir. June, 1996. These techniques enable the read margin to be increased. However, these techniques relate to dynamic RAMs (DRAMs). Accordingly, if they are applied directly to an MRAM, the problem described below may occur. When these techniques are applied to an MRAM, a voltage amplified by overdriving read bit lines is applied to the MTJ element. As a result, the insulating film in the MTJ element may be degraded or damaged. This problem has come more marked particularly because MRAMs have been miniaturized to reduce the thickness of the insulating film in the MTJ element and thus the insulating withstand voltage (for example, 10 [MV/cm]).